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Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction in eukaryotes, such as animals, plants and fungi. The number of sets of chromosomes in the cell undergoing meiosis is reduced to half the original number, typically from two sets (diploid) to one set (haploid). The cells produced by meiosis are either gametes (the usual case in animals) or otherwise usually spores from which gametes are ultimately produced (the case in land plants). In many organisms, including all animals and land plants (but not some other groups such as fungi), gametes are called sperm in males and egg cells or ova in females. Since meiosis has halved the number of sets of chromosomes, when two gametes fuse during fertilization, the number of sets of chromosomes in the resulting zygote is restored to the original number.
Meiotic division occurs in two stages, meiosis I and meiosis II, dividing the cells once at each stage. The first stage begins with a diploid cell that has two copies of each type of chromosome, one from each the mother and father, called homologous chromosomes. All homologous chromosomes pair up and may exchange genetic material with each other in a process called crossing over. Each pair then separates as two haploid cells are formed, each with one chromosome from every homologous pair.
In the second stage, each chromosome splits into two, with each half, called a sister chromatid, being separated into two new cells, which are still haploid. This occurs in both of the haploid cells formed in meiosis I. Therefore from each original cell, four genetically distinct haploid cells are produced. These cells can mature into gametes
Meiosis begins with one diploid cell containing two copies of each chromosome—one from the organism's mother and one from its father. The cell divides twice, potentially producing up to four haploid cells containing one copy of each chromosome. ("Potentially" because in some cases, such as the formation of oocytes in mammals, only one of the possible four haploid cells survives.) In animals the haploid cell resulting from meiosis is a male or female gamete. Each of the resulting chromosomes in the gamete cells is a unique mixture of maternal and paternal DNA, resulting in offspring that are genetically distinct from either parent. This gives rise to genetic diversity in sexually reproducing populations. This genetic diversity can provide the variation of physical and behavioural attributes (phenotypes) upon which natural selection can act.
It is also noteworthy that during meiosis, specific genes are more highly transcribed, and these are called the meiome, the term used in functional genomics for the meiotic transcriptome.[1][2] Meiosis is a key feature for all sexually reproducing eukaryotes in which homologous chromosome pairing, synapse and recombination occur. In addition to strong meiotic stage-specific expression of mRNA (the meiome), however, there are also pervasive translational controls (e.g. selective usage of preformed mRNA), regulating the ultimate meiotic stage-specific protein expression of genes during meiosis.[3] Thus, both the meiome and translational controls determine the broad restructuring of meiotic cells needed to carry out meiosis.
Prior to the meiosis process the cell's chromosomes are duplicated by a round of DNA replication, creating from the maternal and paternal versions of each chromosome (homologus) two exact copies, sister chromatids, attached at the centromere region. In the beginning of meiosis, the maternal and paternal homologs pair with each other. Then they typically exchange parts by homologous recombination leading to crossovers of DNA between the maternal and paternal versions of the chromosome. Spindle fibers bind to the centromeres of each pair of homologs and arrange the pairs at the spindle equator. Then the fibers pull the recombined homologs to opposite poles of the cell. As the chromosomes move away from the center the cell divides into two daughter cells, each containing a haploid number of chromosomes composed of two chromatids.
After the recombined maternal and paternal homologs have separated into the two daughter cells, a second round of cell division occurs. There meiosis ends as the two sister chromatids making up each homolog are separated and move into one of the four resulting gamete cells. Upon fertilization, for example when a sperm enters an egg cell, two gamete cells produced by meiosis fuse. The gamete from the mother and the gamete from the father each contribute one half of the set of chromosomes that make up the new offspring's genome.
Meiosis uses many of the same mechanisms as mitosis, the type of cell division used by eukaryotes like plants and animals to split one cell into two identical daughter cells. In all plants and in many protists meiosis results in the formation of spores: haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, like bdelloid rotifers, do not have the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis.
Meiosis does not occur in archaea or bacteria, which generally reproduce via asexual processes such as binary fission. However, a similar "sexual" process, known as bacterial transformation, involves transfer of DNA from one bacterium to another and recombination of these DNA molecules of different parental origin.